Ultrasonic medical device operating in a transverse mode

ABSTRACT

An ultrasonic medical device comprises an ultrasonic vibration generator that generates vibration along its longitudinal axis. The ultrasonic vibration is transmitted through an ultrasonic coupler and a series of transformer sections that amplify the ultrasonic vibration. A flexible member is coupled to the distal end of the transformer sections, and is thus supplied with a longitudinal vibration at its base by the transformer sections. The flexible member is designed so that it converts the longitudinal vibration into a standing wave that runs along the length of the flexible member. The standing wave produces a series of nodes and anti-nodes along the length of the flexible member. Each of the anti-nodes produces cavitation in fluids in contact with the probe. The cavitation of the fluids causes destruction of adjacent tissue. In this manner, the entire length of the flexible member becomes a working surface that may be utilized for destroying tissue.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to medical devices, andmore particularly to an ultrasonic medical device for destroying tissuein a controlled fashion within a human body.

[0003] 2. Description of Related Art

[0004] Medical instruments utilizing ultrasonic energy to destroy tissuein a human body are known in the art. One drawback of existingultrasonic medical instruments which remove tissue is that typicallydoctors have considered them to be slow in comparison to methods such assurgical excision. Part of the reason for this perceived slowness isexplained by the fact that most existing ultrasonic devices rely on alongitudinal vibration of the tip of the probe. In other words, the tipof the probe is vibrated in a direction in line with the longitudinalaxis of the probe. This produces a tissue destroying affect only at thetip of the probe.

[0005] One solution that has been proposed is to vibrate the tip of theprobe in a transverse direction—i.e. perpendicular to the longitudinalaxis of the probe—in addition to vibrating the tip in the longitudinaldirection. For example, U.S. Pat. No. 4,961,424 to Kubota et al.discloses an ultrasonic treatment device to destroy and emulsifyconcretions or tissue in a human body. The Kubota et al. device producesboth a longitudinal and transverse motion at the tip of the probe. TheKubota et al. patent, however, still relies solely on the tip of theprobe to act as a working surface. Therefore, it improves the efficiencyof the tip, but still relies on the tip of the probe to perform allcutting actions.

[0006] Although Kubota et al. describe providing a transverse motion atthe tip of the probe, a transverse motion along the length of the probehas generally been discouraged. For example, U.S. Pat. No. 4,474,180 toAngulo discloses an ultrasonic kidney stone disintegration instrumentwith a damping material applied to the wire probe to inhibit lateralvibrations of the wire in the region of the connection to the ultrasonictransducer.

[0007] Another proposed method of improving the speed of ultrasonictissue remove is oscillating the tip of the probe in addition tolongitudinally vibrating the tip of the probe. For example, U.S. Pat.No. 4,504,264 to Kelman discloses an ultrasonic treatment device whichimproves the speed of ultrasonic tissue removal. In the Kelman device,the tip of the probe is vibrated longitudinally and also oscillated, sothat the cutting efficiency of the probe tip is improved. Again,however, only the tip of the probe performs a cutting action.

BRIEF SUMMARY OF THE INVENTION

[0008] The object of the present invention is to provide an ultrasonicmedical device capable of destroying and emulsifying tissue throughcavitation in the human body with a higher efficiency by means of aflexible probe operating in a transverse mode. As used herein, atransverse mode of operation is used to describe a flexible probe with aplurality of nodes and anti-nodes along the length of the probe.

[0009] In accordance with this object, an ultrasonic medical devicecomprises an ultrasonic vibration generator that generates vibrationalong its longitudinal axis. The ultrasonic vibration is transmittedthrough an ultrasonic coupler and a series of transformer sections thatamplify the ultrasonic vibration. A flexible member is coupled to thedistal end of the transformer sections, and is thus supplied with alongitudinal vibration at its base by the transformer sections. Theflexible member is designed so that it converts the longitudinalvibration into a standing wave that runs along the length of theflexible member. The standing wave produces a series of nodes andanti-nodes along the length of the flexible member. Each of theanti-nodes produces cavitation in fluids in contact with the probe. Thecavitation of the fluids causes destruction of adjacent tissue. Thus, inthis manner, the entire length of the flexible member becomes a workingsurface that may be utilized for destroying tissue.

[0010] Therefore, in contrast to the prior art designs that only utilizea tip of a probe as a surface, the entire length of the flexible memberforms a cutting surface in the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 shows a schematic view of an ultrasonic probe constructedin accordance with the principles of the invention;

[0012]FIG. 2 shows the flexible member of the ultrasonic probe operatingin a transverse mode;

[0013]FIG. 3 shows a probe assembly for use in an ultrasonic probeconstructed in accordance with the principles of the invention;

[0014]FIG. 4 shows a cross-sectional view of the handle assembly of anultrasonic probe constructed in accordance with the principles of theinvention;

[0015] FIGS. 5A-5D show possible various cross-sectional profiles of aflexible member for use in the present invention; and

[0016]FIG. 6 shows the ultrasonic probe and an associated sheath.

DETAILED DESCRIPTION OF THE INVENTION

[0017] As seen in FIGS. 1 and 3, the ultrasonic probe has a handlesection 10 and a probe section 12. The handle is formed by an ultrasonicdriver 14 and an ultrasonic coupling horn 16. The ultrasonic driver hasa longitudinal axis. The driver produces an ultrasonic vibration in therange of 20-80 kHz. The nominal driver amplitude is 50 microns at 100volt peak to peak sinusoidal excitation. The vibration is along thedirection of the longitudinal axis. In the embodiment illustrated, thetransducer is PZT-4. However, the driver can utilize a variety ofmethods to produce an ultrasonic vibration, such as piezoelectric,magnetostrictive, pneumatic, or hydraulic, as are known to those skilledin the art. A control unit (not illustrated) controls the ultrasonicdriver. The control unit allows an operator to adjust the frequency andamplitude of the vibration which is produced by the driver. In theexemplary embodiment illustrated here, the probe is designed to operateat a frequency of 20 kHz. However, the probe may be designed to operateat frequencies in the range of 20 kHz to 80 kHz, as described in detailin the theory of operation section.

[0018] The ultrasonic driver is coupled to a coupling horn 16, and theultrasonic vibration is transmitted from the driver to the couplinghorn. The coupling horn is connected to the probe section 12. The probesection has a series of transformer sections 18, 20. The transformersections are a series of shafts constructed from any suitable material,such as Ti-6Al-4v titanium. The transformer sections transmit vibrationsfrom the coupling horn to a flexible member 22 at the distal end of theprobe section. In the process of transmission, the amplitude of thevibration is amplified by the transformer sections. The diameter of thetransformer sections are chosen so as to produce a suitable amount oflongitudinal vibration at the end of the transformer section. The gainof the transformer sections is controlled by the ratio of the area ofthe sections. In the exemplary embodiment described herein, thetransformer sections are designed to produce a gain of about 4-5 overthe transducer. This is achieved by setting the diameter of thetransformer sections 18, 20 at 0.150 and 0.080 inches, respectively. Thelength of the transformer sections 18, 20 are 1.500 and 7.554 inches,respectively. The transformer section 18 has a threaded portion 24 tomate with the coupling horn 16, and has a portion 26 which is adapted sothat it may be grasped with a wrench or another tool to tighten theconnection.

[0019] A flexible member 22 is attached to the end of the lasttransformer section, and is driven by the last transformer section. Theflexible member is a thin, wire like probe, typically less than 1 mm indiameter. In the embodiment shown, the flexible member has a circularcross-section with a diameter of 0.020 inches. The flexible member mayhave other cross-sections, such as a rectangular or oval cross-section.The flexible portion of the probe can be multiple wavelengths in length.In the embodiment shown, the flexible member is 4.046 inches long, whichcorresponds to a device operating with a frequency of approximately 20kHz. The preferred material is 6Al-4v titanium; however, any othermaterials may be used as long as the operating parameters fall withinthe operation limits set by the strength of the material, as discussedin detail below.

[0020]FIG. 4 shows a cross-sectional view of the handle assembly of anultrasonic probe constructed in accordance with the principles of theinvention. FIG. 4 shows an exemplary horn assembly 400 suitable for usein the present invention. A housing 402 has a end cap 404 and a rearportion 406. The end cap 404 has an internal threaded portion 408 whichmates with an external threaded portion 410 of the rear portion 406. Therear portion 406 of the housing 402 has a recess 412 which receives anextended portion 414 of the horn 416. The end cap 404 is shaped as aring with a opening 418. The horn 416 fits through the opening 418. Aflange 420 on the horn 416 is larger than the opening 418 so that whenthe end cap 404 is screwed on, the horn 416 is held tightly into thehousing 402.

[0021] The horn 416 has female threads 424 at one end 426 to mate with aprobe assembly. Grooves 438 are provided on the horn 416. O-rings (notshown) may be placed into the grooves to provide a substantiallyfluid-tight seal.

[0022] A stack of piezo-ceramic drivers 428 is arranged around the horn416. The driver stack 428 has four driver ceramics 430, and anadditional feedback ceramic 432. The feedback ceramic 432 is used tomeasure the driver amplitude. Each driver is provided with a nickelelectrode for connection to an electrical source (not shown). Theelectrical source provides an alternating waveform at the appropriatefrequency and amplitude. Insulators 434 are provided to isolate thepiezo-ceramics from the horn and from the feedback ceramic. MACOR™insulators, available from Corning, are one suitable type of insulator.The extended portion 414 of the horn 416 is threaded so that a nut 436may be used to secure the piezo-ceramic drivers to the horn.

[0023] FIGS. 5A-5D show various cross-sectional profiles of ultrasonicprobes which are suitable for use with the present invention. As will bediscussed in detail later, any cross-sectional profile may be utilizedso long as certain design constraints are met.

[0024] As seen in FIG. 6, proximal to the desired, active length 606 ofthe probe 608, the probe 608 is placed within a sheath 600 which canprovide irrigation channels and aspiration channels 602, 604. Irrigationis preferably provided between the probe 608 and the sheath 600. Thesheath 600 is preferably made of PTFE or Teflon tubing so as to absorbthe ultrasonic energy emanating from portions of the probe locatedwithin the sheath, thereby allowing control over the amount of tissueaffected by the probe. The sheathing is not restricted to the preferredmaterials as long as it is made of a material which is not heated by theultrasonic energy, although the irrigation fluid can be used to cool thesheath material. The probe may be extended or retracted from the sheathto modify the amount of probe exposed, thereby modifying the activelength 606 of the probe. Further details regarding one suitable sheathare described in Applicant's co-pending application S/No. 60/157,824,which is hereby incorporated by reference.

[0025] Theory of Operation of the Invention

[0026] Although, not intended to be bound by the following theory ofoperation, it is believed that the following theory describes theoperation of the ultrasonic probe of the present invention. Inoperation, the longitudinal push delivered by the transformer sectionscauses a flexing or buckling of the thin member at the end of the probe.The buckling may be realized as a flexure or standing transverse wavealong the length of the probe section. The flexure case is simply thefirst order transverse mode of vibration as will be described below.

[0027] In a fluid or fluid containing medium, each of the anti-nodes(positions corresponding to maximum transverse displacement) along thelength of the probe cause cavitation of the fluid in a directionperpendicular to the longitudinal axis of the probe.

[0028] Cavitation is a void or bubble produced by the inability of thefluid to overcome the stresses induced by the motion of the probe. Thecollapse of the cavitation bubbles in and around cellular (orbiological) material produces a shockwave that erodes or fragments thematerial allowing it to be removed through aspiration and suction. Themechanism of cavitation and its affect on tissues is well known in theart, and is described in such literature as U.S. Pat. No. 3,526,219 toBalamuth.

[0029] The equations of motion governing the operation of the areobtained by applying Newton's second law to the forces and accelerationsacting upon an infinitesimal segment. The equation of motion for thetransverse oscillations of a thin member (neglecting losses in thematerial and surroundings) is then given by: $\begin{matrix}{{\frac{\partial^{4}\xi}{\partial X^{4}} + {\frac{1}{\left( {\kappa \quad c} \right)^{2}}\frac{\partial^{2}\xi}{\partial t^{2}}}} = 0} & 2.0\end{matrix}$

[0030] Where x is the distance along the flexible portion, t is the timein seconds ξ is the transverse displacement, κ is the radius ofgyration, and c is the speed of sound in the material.

[0031] In can be shown for boundary conditions which assume a flexiblemember of length l fixed at one end and free at the other, the generalsolution to this equation will have the form: $\begin{matrix}{\xi = {{\cos \left( {{\omega \quad t} + \varphi_{n}} \right)}\left( {{A\left( {{\cosh \frac{\omega \quad X}{v}} - {\cos \frac{\omega \quad X}{v}}} \right)} + {B\left( {{\sinh \frac{\omega \quad X}{v}} - {\sin \frac{\omega \quad X}{v}}} \right)}} \right)}} & 2.1\end{matrix}$

[0032] Applying the boundary conditions it can be shown that$\begin{matrix}{{{\cot \left( \frac{\omega \quad l}{2v} \right)}{\tanh \left( \frac{\omega \quad l}{2v} \right)}} = 1} & 2.2\end{matrix}$

[0033] Where ω is the angular frequency in radians per second, x is thedistance along the flexible member (as before) and v is the phasevelocity given by:

ν={square root}{square root over (ωcκ)}  2.3

[0034] Here c is the longitudinal propagation velocity given by:

c={square root}{square root over (Y/ρ)}  2.4

[0035] where Y is Young's modulus and ρ is the density of the material.

[0036] The solutions of equations 2.2 only occur at discretefrequencies, which for the first four overtones can be shown to be:$\begin{matrix}{f_{n} = \frac{\pi \quad c\quad \kappa \quad A_{n}}{8l^{2}}} & 2.5\end{matrix}$

[0037] The A_(n) terms are the solutions to equation 2.2. For the nthovertone they are (1.194)², (2.988)², (5)², (7)² . . . (2n−1)²

[0038] For overtones of the fundamental the node positions along theflexible member can be derived from the general solution given inequation 2.1 The nodal positions are the points at which thedisplacements and the bending moment are zero:${\xi_{n} = 0},{\frac{\partial^{2}\xi_{n}}{\partial X^{2}} = 0}$

[0039] with $\begin{matrix}{\frac{\partial^{2}\xi_{n}}{\partial X^{2}} = {{\cos \left( {{\omega \quad t} + \varphi} \right)}\left( \frac{\omega}{v} \right)^{2}\left( {{A\left( {{\cosh \frac{\omega \quad X}{v}} + {\cos \frac{\omega \quad X}{v}}} \right)} + {B\left( {{\sinh \frac{\omega \quad X}{v}} + {\sin \frac{\omega \quad X}{v}}} \right)}} \right)}} & 2.6\end{matrix}$

[0040] Using equations 2.1 and 2.6 it can be shown that: $\begin{matrix}{{\tan \frac{\omega \quad X}{v}} = {\tanh \frac{\omega \quad X}{v}}} & 2.7\end{matrix}$

[0041] which has solutions for: $\begin{matrix}{\frac{\omega \quad}{v} = {\frac{\pi}{2l}\left( {5,7,{9\quad \ldots}}\quad \right)}} & 2.8\end{matrix}$

[0042] The positions of the nodes for a member of length l will then be:

[0043] First overtone: x=0

[0044] Second overtone x=0, x=0.7741

[0045] Third overtone x=0, x=0.51, x=0.8681

[0046] Fourth overtone x=0, x=0.3561, x=0.6441, x=0.9051

[0047] Etc.

[0048] Figure Two shows the flexible portion oscillating in modes up tothe fourth overtone.

[0049] For a practical design the forces acting on the flexible memberhave to be kept within safe limits for the material chosen. The bendingmoment of the flexible member is given by the equation: $\begin{matrix}{M = {{YA}\quad \kappa^{2}\frac{\partial^{2}\xi}{\partial x^{2}}}} & 3.0\end{matrix}$

[0050] with A being the cross sectional area of the flexible member.Equation 3.0 will be recognized immediately as the standard differentialequation for a beam in flexure.

[0051] The shear force acting along the member will be given by theequation: $\begin{matrix}{F_{s} = {\frac{\partial M}{\partial x} = {{YA}\quad \kappa^{2}\frac{\partial^{3}\xi}{\partial x^{3}}}}} & 3.1\end{matrix}$

[0052] The preferred embodiment is a probe of circular cross section asdescribed; however alternate shapes could be used as long as certaindesign constraints are considered. The key parameter is the Yκ² termappearing in equations 3.0 and 3.1, often referred to as the flexuralstiffness. For annealed Ti-6AL-4V titanium optimal values are in therange 2.5×10⁷ to 8.5×10⁷ N/m. Note that the use of the flexuralstiffness as a design parameter allows a shape independent specificationfor flexible member.

[0053] The driver and transformer sections are designed to providesufficient longitudinal amplitude to support the desired transverse modeamplitude (see section on design constraints below). Typically thehandle and probe assembly are designed to support a longitudinalamplitude which will be sufficient to induce buckling in the flexiblemember. The length of the entire probe and handle assembly is chosen toplace a longitudinal anti-node at the end of the flexible member. Thisrestricts the length of the handle and tip assembly to integer multiplesof one half the longitudinal wavelength. In actual practice it has beenfound that a slight de-tuning of around 3 to 5 percent aids theconversion to the transverse mode. It should be noted that there is nolongitudinal vibration of the tip as this is converted entirely into atransverse vibration through buckling of the thin member at the tip.

[0054] The force, or longitudinal push, imparted to the flexible memberby the longitudinal section must be sufficient to induce buckling. Themaximum longitudinal force exerted at startup must meet the Eulerconditions for buckling, which are the solutions to equation 3.0,yielding the formula for the critical force: $\begin{matrix}{{P_{crit} = \frac{n^{2}\pi^{2}Y\quad \kappa^{2}}{l^{2}}},\left( {{n = 1},2,{3\quad \ldots}} \right)} & 3.2\end{matrix}$

[0055] For the longitudinal drive the maximum stress at startup will be:$\begin{matrix}{s = \frac{2\pi \quad Y\quad f\quad \xi_{m}}{c}} & 3.3\end{matrix}$

[0056] Where ξ_(m) is the maximum longitudinal displacement of theassembly (probe and handle), f is the drive frequency, c is thelongitudinal propagation velocity (Eq. 2.4) and Y is Young's modulus forthe material.

[0057] An optimal design will try to place as many anti-nodes aspossible along the length of the flexible member. In the exemplaryembodiment described and illustrated before, with a 3.748 inch longflexible member with a diameter of 0.020 inches, 6 nodes are produced ata frequency of 20 kHz.

[0058] The proceeding equations show that the stresses on the materialincrease with frequency. When coupled with the need to producesufficient amplitude to remove tissue upper bounds for frequency can beestablished. To produce cavitation in fluid the transverse amplitudeshould be at least 75 microns. This will limit the frequency to about 80kHz for 6Al-4V titanium (this disregards material losses which must beexperimentally determined). The lower limit for the frequency is usuallychosen to be outside of the range of human hearing, or greater than 20kHz.

[0059] The transverse mode probe is much more effective at tissueremoval than are the longitudinal designs of the prior art. One reasonfor this is because the action of the energy is along most of the lengthof the exposed flexible member and is not confined to the surface areaof the tip of the member. The probes described in the prior art whichare only driven in the longitudinal direction only work at the tip. Evenwith a solid tip, its active area in contact with tissue is much lessthan the transverse mode tip. Also, the tissue destruction of thetransverse mode probes extends up to 1 mm circumferentially beyond theprobe. The following calculations indicate the efficacy of thetransverse mode compared to a standard longitudinal probe.

[0060] A rigid, solid, 4 mm probe works only at the tip. As it movesforward and back, it cavitates the fluid in front of it. The volume oftissue effected is: Frequency f 20,000 hz Stroke Δx 350 microns (.35 mm)Radius r 2 mm Cross sectional area A_(x) πr2 12.6 mm² Volume of tissueremoved V A_(x)*Δx 4.40 mm³ per stroke Volume of tissue removed V_(t)V*f/60/1,000 1.47 cc/min per time

[0061] For a 2 cm long by 0.5 mm diameter probe working in thetransverse mode: Frequency f 20,000 hz Radius r 0.25 mm Effective radiusr_(e) 1.25 mm Effective length L 20 mm Cross sectional area A_(x) πr_(e) ² 4.91 mm² Volume of tissue removed per stroke V A_(x)* L 98.1 mm³Volume of tissue removed per time V_(t) V*f/60/1,000 32.7 cc/min

[0062] This means that in these circumstances, the transverse mode tipremoves tissue at a rate 22.2 times faster than the solid tip working inthe longitudinal mode. Also, the transverse mode flexible member istypically ⅛^(th) the size of the longitudinal probe. Comparing two, 0.5mm probes, one working in the longitudinal mode and one in thetransverse mode, the transverse mode tip removes tissue 1,428 timesfaster than the longitudinal probe.

[0063] The transverse mode probe is capable of maintaining its vibrationwhen bent if the sum of the stresses imposed by the transverse vibrationand the bending stresses do not exceed the elastic limit of thematerial. This offers significant advantages over longitudinal modedesigns that are typically rigid over their entire length.

What is claimed is:
 1. An ultrasonic medical device comprising: anultrasonic generator for producing an ultrasonic vibration in adirection along a longitudinal axis of the ultrasonic generator; anultrasonic coupling horn; at least one transformer sectionultrasonically coupled to the ultrasonic source by the ultrasoniccoupling horn, the transformer section modifying the amplitude of theultrasonic vibration; and a flexible member driven by the transformersection, wherein the flexible member operates in a transverse mode ofoperation to produce a plurality of nodes and anti-nodes along thelength of the flexible member.
 2. A device according to claim 1, whereinthe ultrasonic generator produces an ultrasonic vibration in the rangeof 20-80 khz.
 3. A device according to claim 1, wherein the ultrasonicgenerator produces an ultrasonic vibration of approximately 20 khz.
 4. Adevice according to claim 3, wherein a length of the flexible member ischosen so that eight nodes are produced along the length of the flexiblemember.
 5. A device according to one of claims 1-4, wherein the flexiblemember is a thin, flexible member capable of being deflected andarticulated when the device is in operation.
 6. A device according toclaim 5, wherein at least one transformer section is formed of one ofthe following materials: titanium, aluminum, or steel.
 7. A deviceaccording to one of claims 1-4, wherein the flexible member is formed ofone of the following materials: titanium, aluminum, or steel.
 8. Adevice according to one of claims 1-4, wherein the transformer sectionis sized so that it produces a gain of about 4-5 over the transducer. 9.A device according to claim 1, wherein the flexible member has acircular cross-section.
 10. A device according to claim 9, wherein theflexible member has a diameter of less than 1 mm.
 11. A device accordingto claim 9, wherein the flexible member has a diameter of 0.020 inches.12. A device according to claim 9, wherein the flexible member has adiameter of 0.030 inches.
 13. A device according to claim 1, wherein theflexible member has a square cross-section.
 14. A device according toclaim 1, wherein the flexible member has a rectangular cross-section.15. A device according to claim 1, wherein the flexible member has anelliptical cross-section.
 16. A device according to claim 1, wherein theflexural stiffness of the flexible member is in the range 2.5×10⁷ to8.5×10⁷ N/m.
 17. A method of removing tissue from a cavity in a humanbody, comprising the steps of: (a) providing a flexible member with aproximal end, a distal end and a longitudinal axis; (b) providing anultrasonic vibration to the proximal end of the flexible member, theultrasonic vibration being along the longitudinal axis of the flexiblemember; (c) sweeping the flexible member through tissue so that itemulsifies tissue; wherein the ultrasonic vibration creates a standingtransverse wave in the flexible member so that a plurality of nodes andanti-nodes are formed along the length of the flexible member.
 18. Amethod according to claim 17, wherein the ultrasonic generator producesan ultrasonic vibration in the range of 20-80 khz.
 19. A methodaccording to claim 17, wherein the ultrasonic generator produces anultrasonic vibration of approximately 20 khz.
 20. A method according toclaim 19, wherein a length of the flexible member is chosen so thateight nodes are produced along the length of the flexible member.
 21. Amethod according to one of claims 17-20, wherein the flexible member isa thin, flexible member capable of being deflected and articulated. 22.A method according to claim 17, wherein the flexible member is formed ofone of the following materials: titanium, aluminum, or steel.
 23. Amethod according to claim 17, wherein the flexible member has a circularcross-section.
 24. A method according to claim 23, wherein the flexiblemember has a diameter of less than 1 mm.
 25. A method according to claim23, wherein the flexible member has a diameter of 0.020 inches.
 26. Amethod according to claim 23, wherein the flexible member has a diameterof 0.030 inches.
 27. A method according to claim 17, wherein theflexible member has a square cross-section.
 28. A method according toclaim 17, wherein the flexible member has a rectangular cross-section.29. A method according to claim 17, wherein the flexible member has anelliptical cross-section.
 30. A method according to claim 17, whereinthe flexural stiffness of the flexible member is in the range 2.5×10⁷ to8.5×10⁷ N/m.
 31. A method according to claim 17, wherein the tissue isemulsified by the mechanism of cavitation.
 32. A method according toclaim 17, wherein the tissue is emulsified by mechanical action.
 33. Amethod according to claim 17, wherein the flexible member has an area oftissue destroying effect greater than a cross-sectional area of theflexible member.
 34. A method of destroying tissue comprising the stepsof: (a) providing a flexible member with a proximal end, a distal tip,and a longitudinal axis; (b) applying an ultrasonic vibration to theproximal end of the flexible member, the ultrasonic vibration being inthe direction of the longitudinal axis; (c) generating a series of nodesand anti-nodes along the the flexible member so that there issubstantially no longitudinal motion of the distal tip; (d) placing theflexible member in communication with tissue so that the flexible memberdestroys the tissue.
 35. A method according to claim 34, wherein theflexible member is placed into direct contact with the tissue so thatthe tissue is destroyed by mechanical action.
 36. A method according toclaim 34, wherein the flexible member communicates with the tissuethrough a fluid, and the flexible member causes cavitation whichdestroys the tissue.
 37. A method of treating tissue comprising stepsof: (a) providing a flexible member with a longitudinal axis, a distalend, and a proximal end; (b) generating a standing transverse wave inthe flexible member so that a plurality of nodes and anti-nodes areformed along the flexible member; and (c) placing the flexible memberinto communication with tissue so that the tissue is destroyed.
 38. Amethod according to claim 37, wherein the flexible member directlycontacts the tissue so that the tissue is destroyed by mechanicalaction.
 39. A method according to claim 37, wherein the flexible membercauses cavitation in a fluid in contact with the tissue so that thetissue is destroyed by cavitation.
 40. A method according to one ofclaims 37-39, wherein the step of generating a standing wave in theflexible member is accomplished by providing an ultrasonic vibration tothe proximal end of the flexible member.
 41. A method according to claim40, wherein the provided ultrasonic vibration is solely along thelongitudinal axis of the flexible member.
 42. A method according toclaim 41, wherein there is substantially no longitudinal motion of thedistal end of the flexible member.
 43. A method according to claim 40,wherein the ultrasonic vibration has a frequency in the range of 20 kHzto 80 kHz.
 44. A method according to one of claims 37-39, wherein thereis substantially no longitudinal motion at the distal end of theflexible member.
 45. A method of operating an ultrasonic medical device,comprising the steps of: (a) providing a flexible member with a proximalend, a distal end, and a longitudinal end; (b) providing an ultrasonicgenerator to apply an ultrasonic vibration to the proximal end of theflexible member, the ultrasonic vibration being along the longitudinalaxis of the flexible member; and (c) controlling the amplitude andfrequency of the ultrasonic vibration to produce a standing transversewave in the flexible member.
 46. A method according to claim 45, whereinthe frequency is in the range of 20 kHz to 80 kHz.
 47. A methodaccording to claim 46, wherein the amplitude is in the range of 150-350microns.
 48. A method according to claim 45, wherein the amplitude is inthe range of 150-350 microns.
 49. A method according to one of claims45, 46, 47 or 48, further comprising the step of: (d) placing theflexible member in communication with tissue so that the tissue isdestroyed.
 50. A method according to claim 49, wherein the flexiblemember is placed directly into contact with the tissue so that thetissue is destroyed by mechanical action.
 51. A method according toclaim 49, wherein the flexible member is placed into contact with fluidin contact with the tissue so that the tissue is destroyed bycavitation.
 52. A method according to one of claims 45, 46, 47 or 48wherein there is substantially no longitudinal motion at the distal endof the flexible member.
 53. An ultrasonic device comprising: a flexiblemember with a longitudinal axis, a proximal end, and a distal end; andan ultrasonic generator coupled to the proximal end of the flexiblemember, the generator creating ultrasonic vibrations in the direction ofthe longitudinal axis of the flexible member, wherein a length and across-section of the flexible member are sized so that the ultrasonicvibrations are converted into a standing transverse wave with aplurality of nodes and anti-nodes along the flexible member, and thereis substantially no motion along the longitudinal axis at the distal endof the flexible member.
 54. A device according to claim 53, furthercomprising: a series of transformer sections located between theultrasonic generator and the flexible member, the transformer sectionsmodifying the amplitude of the ultrasonic vibrations.
 55. A deviceaccording to one of claims 53 or 54, further comprising: a controldevice connected to the ultrasonic generator to control the frequencyand amplitude of the generated vibrations.
 56. A device according to oneof claims 53 or 54, further comprising a sheath surrounding thetransformer sections and a portion of the flexible member.
 57. A deviceaccording to claim 56, wherein the sheath includes irrigation channels.58. A device according to claim 56, wherein the sheath includesaspiration channels.
 59. A device according to claim 56, wherein thesheath includes irrigation and aspiration channels.
 60. A deviceaccording to claim 56, wherein the sheath and the flexible member areaxially displaceable with one another so that a varying number of nodesare exposed.